1. Technical Field
The present invention relates to an electrode for a semiconductor chip, a semiconductor chip with the electrode, and a method of manufacturing such semiconductor chip.
2. Related Art
For a so-called next-generation DVD such as a Blu-ray disk or a High-Definition Digital Versatile Disc (hereinafter, HD-DVD), laser beam of 405 nm or so in wavelength is employed for retrieving data recorded on the disk surface and writing new data thereon. For such purpose, a semiconductor laser based on a semiconductor crystal predominantly containing gallium nitride (hereinafter, GaN) is employed as the light source of the laser beam.
In a semiconductor laser of a shorter oscillation wavelength, the active layer has a wider band gap and hence a larger built-in potential, and therefore a higher voltage has to be applied for supplying a current. Also, the semiconductor laser typically has a double-hetero structure which encourages efficient reunion of electrons and holes in the active layer. Accordingly, the semiconductor laser includes a clad layer having a wider band gap than the active layer.
In a semiconductor in general, the higher the band gap is, the higher the electric resistance becomes. Therefore, normally the semiconductor laser of the shorter wavelength has the higher electric resistance in the clad layer in the same way.
Further, the semiconductor laser includes metal electrodes disposed in contact with the p-type semiconductor and the n-type semiconductor respectively. The contact resistance between the electrode and each of the semiconductors tends to be higher, in the semiconductor having the wider band gap. For such reasons, higher working voltages are required for supplying the same current, in the ascending order of the laser for a Compact Disc (hereinafter, CD) (wavelength 780 nm), for a DVD (wavelength 650 nm), and for a next-generation DVD (wavelength 405 nm).
A recording and reproducing apparatus for the next-generation DVD also includes the lasers for reading and writing in conventional media such as the CD and the DVD, in addition to the laser for the next-generation DVD such as the blue ray disk and the HD-DVD. Among those lasers, the one for the next-generation DVD requires a prominently higher working voltage, and therefore it is desirable to lower the working voltage as much as possible, to thereby employ a common power source. For this purpose, research and development of the metal electrode are persistently being made, for attaining a lower contact resistance with the p-type semiconductor layer and the n-type semiconductor layer.
Japanese Laid-open patent publication No. H09-8407 proposes a GaN-based semiconductor light emitting element in which the n-side electrode is constituted of Ti/Nb/Au layered in this order from the side of the semiconductor chip.
Also, Japanese Laid-open non-patent publication No. 2005-26291 proposes a GaN-based semiconductor light emitting element in which the n-side electrode is constituted of Pd/Mo/Au layered in this order from the side of the semiconductor chip.
The non-patent document (“Interfacial reactions of Ti/n-GaN contacts at elevated temperature”, Lu C. J. et al, Journal of Applied Physics, 94, 1, (2003) pp. 245-253) provides observational data of interfacial reaction that takes place on the contact interface between Ti and the n-type GaN when heated up to 700° C. or so.
The foregoing techniques, however, still have a room for improvement in the following aspects.
Regarding the n-type GaN-based semiconductor, it is known that a relatively low contact resistance can be attained by employing a metal of a small work function such as Ti, V, or Nb as the contact electrode disposed in contact with the n-GaN layer. In the semiconductor laser, however, the contact resistance may deteriorate from the initial value owing to current supply or heating.
Moreover, Lu C. J. et al reports that heating provokes diffusion of Ga from the n-GaN layer toward the metal electrode. In the GaN a Ga defect is produced after the diffusion of Ga, and such Ga defect often acts as a p-type dopant. Accordingly, the contact resistance between the n-GaN layer and the metal electrode is increased. The increase in contact resistance due to supplying current is also partly because of the emergence of the defect originating from the Ga diffusion.
According to Lu C. J. et al, the heating also provokes diffusion of the nitrogen atom in the n-GaN layer. The N defect tends to act as an n-type dopant, and hence the contact resistance between the n-type semiconductor layer and the electrode may be expected to decrease. In practical operation of the semiconductor device, however, the contact resistance is actually prone to deteriorate because the emergence of the Ga defect or void exerts a greater effect toward increasing the resistance.
Such drawback is an issue to be resolved in common to the semiconductor chips that include the contact electrode disposed on the n-type semiconductor layer in contact therewith, without limitation to the nitride semiconductor chip including the n-GaN layer.